It is known that powdery substances can be compressed by using a vacuum and mechanical pressure on rotating, gas-permeable surfaces. The entire filter surface which is not used for mechanical compression or covered with mechanically compressed material moves within a closed housing in the material to be compressed and transports the material to the compressing area. Prior to the compression, the layer density of the non-compressed material is adjusted on the roller by means of a scraper.
The compressed material is removed from the rollers by means of stripping devices and falls down for packaging (see for example German Patent Specification No. DE-AS 11 29 459).
In the case of rollers with a fabric cover, the scraper must maintain a minimum clearance from the roller surface. The scraper, for example, can not completely strip off silicic acid which has been compressed on such an apparatus.
The residual layer which remains is extremely disadvantageous. For example, it causes a not inconsiderable amount of transported material to be retained, and, in addition, the pressure difference on the rollers is so sharply reduced that the degree of compression decreases.
This known method has the disadvantage that a very strong compression is necessary to compensate for the loss of compression due to the loosening of the product on the way from the compression device to the packing machine. In order to obtain a certain degree of compression in the packaging, it is therefore necessary to apply a considerably greater compression in the compressing device. This creates the risk of losing, as a result of the great compression, the properties of the pyrogenically prepared silicic acid which render it suitable for various applications.
It is known that acetylene black can be compressed by using several pressing rollers and a circulating pressing band in conjunction with the vacuum of a rotating roller designed as a rotary vacuum drum.
The acetylene black is supplied freely flowing from above to a tapering intake slot between a rotating rotary drum provided with filter cloth and sieve jacket and under a vacuum in this section and between a rotating intake roller with circulating air-permeable, elastic pressing band, is pre-compressed and is compressed between the rotating rotary drum likewise under a vacuum in this section and between the following, elastic, nonair-permeable pressing band in conjunction with the mechanical pressure of rotating pressing rollers acting on the pressing band (DDR-PS 43981).
This known method has the disadvantage that it can not be used for the compression of pyrogenically prepared silicic acid.
Moreover, pyrogenically prepared silicic acid in a mixture with air behaves like a thin liquid, in contrast to acetylene black. Therefore, feeding of freely-flowing pyrogenic silicic acid into the intake slot of this known device causes the pyrogenic silicic acid to flow out of the device like water. A controlled compression is therefore not possible.